Beyond Words: Natural Language Sampling of Lexical and Non-Word Vocalizations in Preschool Children with Autism, Down Syndrome, and Fragile X Syndrome

Purpose: This study examined whether word-based and non-word natural language sampling (NLS) measures complement standardized language scores in characterizing expressive communication among preschool children with autism with language impairment (AUT-LI), Down syndrome (DS), and fragile X syndrome (FXS). Method: Participants included 82 preschool children with AUT-LI (n = 32), DS (n = 34), or FXS (n = 16). Expressive communication was characterized using the Preschool Language Scales and 10-minute natural language samples collected during a standardized play-based protocol. NLS measures included traditional word-based metrics, such as lexical diversity, intelligibility, and word approximations, as well as tiered non-word vocalization measures, including non-speech, single-phoneme, and multi-phoneme vocalizations. Analyses examined age associations, diagnostic group differences, between group differences in variability, and exploratory communication profiles derived using principal component analysis and partitioning around medoids clustering. Results: Diagnostic groups did not significantly differ on standardized expressive language scores. Children with FXS showed higher performance than the AUT-LI and DS groups on several word-based NLS measures, whereas non-word vocalization measures primarily revealed differences in within-group variability. Word-based NLS measures were consistently associated with standardized expressive language skills, while associations for non-word vocalizations varied by vocalizations type. Cluster-validity indices factored a two-cluster solution. A hypothesis-informed three cluster solution that included non-word vocalizations suggested Higher Language, Lower Output, and High Output-Low Structure profiles. Conclusions: Standardized and NLS measures provide complementary descriptions of expressive communication in children with neurodevelopmental disorders. Non-word vocalization coding may be especially useful for describing clinical meaningful variation among children with limited expressive language. Keywords: Natural language sampling; expressive language; neurodevelopmental disorders; autism spectrum disorder; Down syndrome; Fragile X syndrome